The adequate management of pain constitutes an important challenge, since currently available treatments provide in many cases only modest improvements, leaving many patients unrelieved (Turk, D. C., Wilson, H. D., Cahana, A.; 2011; Lancet; 377; 2226-2235). Pain affects a big portion of the population with an estimated prevalence of 20% and its incidence, particularly in the case of chronic pain, is increasing due to the population ageing. Additionally, pain is clearly related to comorbidities, such as depression, anxiety and insomnia, which leads to important productivity losses and socio-economical burden (Goldberg, D. S., McGee, S. J.; 2011; BMC Public Health; 11; 770). Existing pain therapies include non-steroidal anti-inflammatory drugs (NSAIDs), opioid agonists, calcium channel blockers and antidepressants, but they are much less than optimal regarding their safety ratio. All of them show limited efficacy and a range of secondary effects that preclude their use, especially in chronic settings.
Voltage-gated calcium channels (VGCC) are required for many key functions in the body. Different subtypes of voltage-gated calcium channels have been described (Zamponi et al., Pharmacol Rev. 2015 67:821-70). The VGCC are assembled through interactions of different subunits, namely α1 (Cavα1), β (Cavβ) α2δ (Cavα2δ) and γ (Cavγ). The α1 subunits are the key porous forming units of the channel complex, being responsible for the Ca2+ conduction and generation of Ca2+ influx. The α2δ, β, and γ subunits are auxiliary, although very important for the regulation of the channel, since they increase the expression of the α1 subunits in the plasma membrane as well as modulate their function, resulting in functional diversity in different cell types. Based on their physiological and pharmacological properties, VGCC can be subdivided into low voltage-activated T-type (Cav3.1, Cav3.2, and Cav3.3), and high voltage-activated L-(Cav1.1 through Cav1.4), N-(Cav2.2), P/Q-(Cav2.1), and R-(Cav2.3) types, depending on the channel forming Cavα subunits. All of these five subclasses are found in the central and peripheral nervous systems. Regulation of intracellular calcium through activation of these VGCC plays obligatory roles in: 1) neurotransmitter release, 2) membrane depolarization and hyperpolarization, 3) enzyme activation and inactivation, and 4) gene regulation (Perret and Luo, Neurotherapeutics. 2009 6:679-92; Zamponi et al., 2015 supra; Neumaier et al., Prog Neurobiol. 2015 129:1-36.). A large body of data has clearly indicated that VGCC are implicated in mediating various disease states including pain processing. Drugs interacting with the different calcium channel subtypes and subunits have been developed. Current therapeutic agents include drugs targeting L-type Cav1.2 calcium channels, particularly 1,4-dihydropyridines, which are widely used in the treatment of hypertension. T-type (Cav3) channels are the target of ethosuximide, widely used in absence epilepsy. Ziconotide, a peptide blocker of N-type (Cav2.2) calcium channels, has been approved as a treatment of intractable pain. (Perret and Luo, 2009, supra; Vink and Alewood, Br J Pharmacol. 2012 167:970-89.).
The Cav1 and Cav2 subfamilies contain an auxiliary α2δ subunit, which is the therapeutic target of the gabapentinoid drugs of value in certain epilepsies and chronic neuropathic pain. To date, there are four known α2δ subunits, each encoded by a unique gene and all possessing splice variants. Each α2δ protein is encoded by a single messenger RNA and is post-translationally cleaved and then linked by disulfide bonds. Four genes encoding α2δ subunits have now been cloned. α2δ-1 was initially cloned from skeletal muscle and shows a fairly ubiquitous distribution. The α2δ-2 and α2δ-3 subunits were subsequently cloned from brain. The most recently identified subunit, α2δ-4, is largely non-neuronal. The human α2δ-4 protein sequence shares 30, 32 and 61% identity with the human α2δ-1, α2δ-2 and α2δ-3 subunits, respectively. The gene structure of all α2δ subunits is similar. All α2δ subunits show several splice variants (Davies et al., Trends Pharmacol Sci. 2007 28:220-8; Dolphin A C, Nat Rev Neurosci. 2012 13:542-55, Biochim Biophys Acta. 2013 1828:1541-9.).
The Cavα2δ-1 subunit may play an important role in neuropathic pain development (Perret and Luo, 2009, supra; Vink and Alewood, 2012, supra). Biochemical data have indicated a significant Cavα2δ-1, but not Cavα2δ-2, subunit upregulation in the spinal dorsal horn, and DRG (dorsal root ganglia) after nerve injury that correlates with neuropathic pain development. In addition, blocking axonal transport of injury-induced DRG Cavα2δ-1 subunit to the central presynaptic terminals diminishes tactile allodynia in nerve injured animals, suggesting that elevated DRG Cavα2δ-1 subunit contributes to neuropathic allodynia.
The Cavα2δ-1 subunit (and the Cavα2δ-2, but not Cavα2δ-3 and Cavα2δ-4, subunits) is the binding site for gabapentin which has anti-allodynic/hyperalgesic properties in patients and animal models. Because injury-induced Cavα2δ-1 expression correlates with neuropathic pain development and maintenance, and various calcium channels are known to contribute to spinal synaptic neurotransmission and DRG neuron excitability, injury-induced Cavα2δ-1 subunit upregulation may contribute to the initiation and maintenance of neuropathic pain by altering the properties and/or distribution of VGCC in the subpopulation of DRG neurons and their central terminals, therefore modulating excitability and/or synaptic neuroplasticity in the dorsal horn. Intrathecal antisense oligonucleotides against the Cavα2δ-1 subunit can block nerve injury-induced Cavα2δ-1 upregulation and prevent the onset of allodynia and reserve established allodynia.
As mentioned above, the α2δ subunits of VGCC form the binding site for gabapentin and pregabalin, which are structural derivatives of the inhibitory neurotransmitter GABA although they do not bind to GABAA, GABAB, or benzodiazepine receptors, or alter GABA regulation in animal brain preparations. The binding of gabapentin and pregabalin to the Cavα2δ subunit results in a reduction in the calcium-dependent release of multiple neurotransmitters, leading to efficacy and tolerability for neuropathic pain management. Gabapentinoids may also reduce excitability by inhibiting synaptogenesis (Perret and Luo, 2009, supra; Vink and Alewood, 2012, supra, Zamponi et al., 2015, supra).
As mentioned before, there are few available therapeutic classes for the treatment of pain, and opioids are among the most effective, especially when addressing severe pain states. They act through three different types of opioid receptors (mu, kappa and gamma) which are transmembrane G-protein coupled receptors (GPCRs). Still, the main analgesic action is attributed to the activation of the μ-opioid receptor (MOR). However, the general administration of MOR agonists is limited due to their important side effects, such as constipation, respiratory depression, tolerance, emesis and physical dependence [Meldrum, M. L. (Ed.). Opioids and Pain Relief: A Historical Perspective. Progress in Pain Research and Management, Vol 25. IASP Press, Seattle, 2003]. Additionally, MOR agonists are not optimal for the treatment of chronic pain as indicated by the diminished effectiveness of morphine against chronic pain conditions. This is especially proven for the chronic pain conditions of neuropathic or inflammatory origin, in comparison to its high potency against acute pain. The finding that chronic pain can lead to MOR down-regulation may offer a molecular basis for the relative lack of efficacy of morphine in long-term treatment settings [Dickenson, A. H., Suzuki, R. Opioids in neuropathic pain: Clues from animal studies. Eur J Pain 9, 113-6 (2005)]. Moreover, prolonged treatment with morphine may result in tolerance to its analgesic effects, most likely due to treatment-induced MOR down-regulation, interalization and other regulatory mechanisms. As a consequence, long-term treatment can result in substantial increases in dosing in order to maintain a clinically satisfactory pain relief, but the narrow therapeutic window of MOR agonists finally results in unacceptable side effects and poor patient compliance.
Polypharmacology is a phenomenon in which a drug binds multiple rather than a single target with significant affinity. The effect of polypharmacology on therapy can be positive (effective therapy) and/or negative (side effects). Positive and/or negative effects can be caused by binding to the same or different subsets of targets; binding to some targets may have no effect. Multi-component drugs or multi-targeting drugs can overcome toxicity and other side effects associated with high doses of single drugs by countering biological compensation, allowing reduced dosage of each compound or accessing context-specific multitarget mechanisms. Because multitarget mechanisms require their targets to be available for coordinated action, one would expect synergies to occur in a narrower range of cellular phenotypes given differential expression of the drug targets than would the activities of single agents. In fact, it has been experimentally demonstrated that synergistic drug combinations are generally more specific to particular cellular contexts than are single agent activities, such selectivity is achieved through differential expression of the drugs' targets in cell types associated with therapeutic, but not toxic, effects (Lehar et al., Nat Biotechnol 2009; 27: 659-666.).
In the case of chronic pain, which is a multifactorial disease, multi-targeting drugs may produce concerted pharmacological intervention of multiple targets and signaling pathways that drive pain. Because they actually make use of biological complexity, multi-targeting (or multi-component drugs) approaches are among the most promising avenues toward treating multifactorial diseases such as pain (Gilron et al., Lancet Neurol. 2013 November; 12(11):1084-95.). In fact, positive synergistic interaction for several compounds, including analgesics, has been described (Schröder et al., J Pharmacol Exp Ther. 2011; 337:312-20. Erratum in: J Pharmacol Exp Ther. 2012; 342:232; Zhang et al., Cell Death Dis. 2014; 5:e1138; Gilron et al., 2013, supra).
Given the significant differences in pharmacokinetics, metabolisms and bioavailability, reformulation of drug combinations (multi-component drugs) is challenging. Further, two drugs that are generally safe when dosed individually cannot be assumed to be safe in combination. In addition to the possibility of adverse drug-drug interactions, if the theory of network pharmacology indicates that an effect on phenotype may derive from hitting multiple targets, then that combined phenotypic perturbation may be efficacious or deleterious. The major challenge to both drug combination strategies is the regulatory requirement for each individual drug to be shown to be safe as an individual agent and in combination (Hopkins, Nat Chem Biol. 2008; 4:682-90.).
An alternative strategy for multitarget therapy is to design a single compound with selective polypharmacology (multi-targeting drug). It has been shown that many approved drugs act on multiple targets. Dosing with a single compound may have advantages over a drug combination in terms of equitable pharmacokinetics and biodistribution. Indeed, troughs in drug exposure due to incompatible pharmacokinetics between components of a combination therapy may create a low-dose window of opportunity where a reduced selection pressure can lead to drug resistance. In terms of drug registration, approval of a single compound acting on multiple targets faces significantly lower regulatory barriers than approval of a combination of new drugs (Hopkins, 2008, supra).
Thus, the present application, relates to the advantages of having dual activity, for μ-receptor and the α2δ-1 subunit of voltage-gated calcium channels, in the same molecule to treat chronic pain.
In this way, the present invention relates to compounds having a complementary dual mechanism of action (μ-receptor agonist and blocker of the α2δ subunit, in particular the α2δ-1 subunit, of voltage-gated calcium channels) which implies a better profile of tolerability than the strong opioids (morphine, oxycodone, fentanyl etc) and/or better efficacy and tolerability than gabapentinoids (pregabalin and gabapentin).
Pain is multimodal in nature, since in nearly all pain states several mediators, signaling pathways and molecular mechanisms are implicated. Consequently, monomodal therapies fail to provide complete pain relief. Currently, combining existing therapies is a common clinical practice and many efforts are directed to assess the best combination of available drugs in clinical studies (Mao, J., Gold, M. S., Backonja, M.; 2011; J. Pain; 12; 157-166).
Accordingly, there is still a need to find compounds that have an alternative or improved pharmacological activity in the treatment of pain, being both effective and showing the desired selectivity, and having good “drugability” properties, i.e. good pharmaceutical properties related to administration, distribution, metabolism and excretion.
The authors of the present invention, have found a series of compounds that show dual pharmacological activity towards both the α2δ subunit, in particular the α2δ-1 subunit, of the voltage-gated calcium channel, and the μ-opioid receptor (MOR or mu-opioid receptor) resulting in an innovative, effective and alternative solution for the treatment of pain.
In view of the existing results of the currently available therapies and clinical practices, the present invention offers a solution by combining in a single compound binding to two different targets relevant for the treatment of pain. This was mainly achieved by providing the compounds according to the invention that bind both to the μ-opioid receptor and to the α2δ subunit, in particular the α2δ-1 subunit, of the voltage-gated calcium channel.